1. Field of the Invention
This invention relates to a toner used for developing positive electrostatic latent images in electrophotography and electrostatic printing, and more particularly to an encapsulated toner suitable for pressure fixation
2. Description of the Prior Art
While many electrophotographic processes have been known as described in U. S. Pat. Nos. 2297691, 3666363 and 4071361 and in other documents, these processes generally comprise forming an electrostatic latent image on a photoconductive layer by some method or other utilizing photoconductive materials, developing the latent image with a toner, and if necessary, transferring the toner image onto a transfer medium such as paper, followed by fixing the transferred image thereupon by applying heat, pressure, or a solvent vapor, to complete a copy.
Various development processes are also known for visualizing such electrostatic latent images by use of a toner, including, for example, the magnetic brush process as described in U.S. Pat. No. 28740603, cascade process as in U.S. Pat. No. 2618552, powder cloud process as in U.S. Pat. No. 2221776, fur brush process, liquid process, and the like.
Toners conventionally used in these development processes are fine powders prepared by dispersing dyes or pigments in natural or synthetic resins. Further, there are known finely powdered toners incorporated with a third material for various purposes.
Developed images are transferred, as required, onto transfer media such as paper and the like. Known methods for fixing toner images include those wherein toner images are heated with a heater, heat roller, or other means to fuse them to the supporting medium; wherein the binder resin in toner images is softened or dissolved in an organic solvent to fix to the supporting medium; and wherein toner images are fixed with pressure to the supporting medium.
In general, a toner is selected so as to fit a given fixing process, and the toner fitting a given fixing process cannot be used in other fixing processes. It is almost impossible, in particular, that a toner for use in the hot melt fixing process so far widely practiced employing a heater is applied to another fixing process such as the heat roller fixing process, solvent fixing process, or pressure fixing process. Accordingly, toners suited for individual fixing processes are under research and development.
The process for fixing a toner with pressure, described in U.S. Pat. No. 3269626 and other documents, has many advantages such as capability of energy saving, no environmental pollution, capability of copying without waiting after the copying machine has been switched on, no danger of scorching a copy, possibility of a high fixing rate, and simplicity of fixing devices available.
This fixing process, however, involves problems in fixabilities of toners and in the offset phenomenon, which is a trouble of transferring toner images to the surface of a pressure roller employed, so that various researches and developments are in progress in order to solve these problems. For example, a pressure-fixable toner containing an aliphatic compound and a thermoplastic resin has been disclosed in U.K. Pat. No. 1210665, an encapsulated pressure-fixable toner having a soft material in the core has been disclosed in U.S. Pat. No. 3788994, and a pressure fixable toner utilizing a block copolymer of a high stiffness polymer component and a flexible polymer component has been disclosed in U.K. Pat. No. 1414159.
However, it has not been successful to obtain a practically useful pressure-fixable toner which is easy to manufacture, has adequate pressure fixability, does not cause the pressure-roller-offset phenomenon, maintains a developing ability and fixability stable to a great number of repeated uses, does not undergo coalescence with the carrier, development metallic sleeve, or photosensitive member surface employed, and has such a good stability as not to agglomerate or cake during storage.
For instance, pressure-fixable toners comprising a soft material, although good in pressure fixability, are difficult to prepare by fine grinding and are liable to cause the pressure-roller-offset phenomenon, to undergo coalescence with the carrier and the photosensitive member surface, and to agglomerate or cake.
On the other hand, rigid resins, although easy to make up into toners and excellent in electrical chargeability and storage stability, are extremely poor in pressure fixability. It is because rigid resins are mostly harder than the cellulose fiber of paper, and hence, when pressed, are merely crushed into the paper without being entangled with the fiber.
Various pressure-fixable encapsulated toner hitherto known involve the following problem: when the core is made of a soft material having good pressure fixability, its deposition on the pressure roller becomes gradually remarkable as the pressure fixing is repeated more and more; this eventually causes the offset phenomenon or the winding of the transfer paper around the pressure roller; if a rigid material is used in the core in order to avoid these phenomena, the pressure fixability will be deteriorated.
Recently, a further process has been put into practice for developing electrostatic latent images with a one-component developer which comprises no carrier but a toner containing fine magnetic particles. In this case, the binder resin in the toner is requested to be satisfactory in miscibility with magnetic particles, adhesion thereto, and impact strength and free flow property of toner particles. It is very difficulst to meet these requirements as well as pressure fixability.